Liquefaction of natural gas



Dec. 22, 1964 Filed June 50, 1958 "31 1 www Bum 1am M w kw M N 5 1.. 9mQN Y B NW \QN ww @m w Qw 1 AW ATTORNEYS United States Patent 3,162,519EQUEFACTION 0F NATURAL GAS Frank M. Peters, Mission, Kane, and .lames DeLory,

Kansas City, Mo, assignors, by inesne assignmentajo Conch InternationalMethane Limited, Nassau, Bahomes, a corporation of the Bahamas Filedlune 3t), 1%8, Ser. No. 74.55%

11 Claims. (ill. 62-23) This invention relates to the liquefaction of agas and, more particularly, to a method and apparatus for theliquefaction of natural gas which is normally composed mostly of methanebut which may contain heavier hydrocarbons, such as ethane, propane,butane and the like, small amounts of aromatic hydrocarbons and variableamounts of non-hydrocarbons such as nitrogen, helium, carbon dioxide,hydrogen sulfide and the like. Illustration of this invention willhereafter be made with reference to the liquefaction of natural gas, butit will be understood that the concepts employed are also capable ofapplication to other low boiling liquefiable gases, such as nitrogen,helium, air, oxygen and the like.

There are many purposes for which natural gas is desired to be reducedto a liquefied state. The main reason resides in the resultantreduction, at equivalent pressure, by about 3 in volume when reducedfrom the gaseous state to a liquefied state, thereby enabling storageand transportation in containers of more economical and practicaldesign.

For example, when gas is transported by pipeline from the source ofsupply to a distant market, it is desirable to operate undersubstantially constant high load factor. Gftentimes the flow capacitywill exceed demand, while at other times the demand may exceed thecapacity of the line. In order to shave oif the peaks where demand wouldexceed supply, it is desirable to store gas when the supply exceedsdemand, whereby peaks in demand can be met by material in storage. Forthis purpose, it is desirable to provide for storage in a liquefiedstate and to vaporize liquid in amounts to meet demand.

Liquefaction of natural gas is of even greater importance in making itpossible to transport the gas from a source of plentiful supply to adistant market where a deficiency exists, especially when the source ofsupply cannot be directly joined with the market by a pipeline or thelike means for the transportation of the gaseous fuel in a gaseousstate. By way of illustration, surplus natural gas is available in theGulf States of the United States, in Venezuela, and in the Persian Gulf,while deficiencies exist in the northern parts of the United States, theEuropean countries, and Japan, yet these sources of supply cannot bejoined by pipeline with some of the markets. Ship transportation in thegaseous state would be uneconorrieal unless the gaseous materials werehighly compressed and then the system would not be commercial because itwould be impractical to provide containers or" suitable strength andcapacity.

it has been determined that natural gas, when shipped from the UnitedStates or Venezuela in large volumes in liquefied state, can be madeavailable in Great Britain, for example, at a price which isconsiderably less than locally manufactured gas. For shipment in largevolume, it is desirable to house the liquefied natural gas in suitableinsulated containers of large capacity at about atmospheric pressure, orpreferably slightly above atmospheric, but not at such high pressures aswould unduly limit the economical capacity of the tank. Depending uponthe amount of higher boiling heavier hydrocarbons present in the naturalgas, the liquefied natural gas will have a boiling point within therange of 240 F. to --258 F. at atmospheric pressure.

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The present invention contemplates a novel method of liquefying gasnormally available at at least a slightly elevated pressure, wherein thegas is first cooled to a liquefied state without substantially reducingthe pressure of the natural gas stream, and then the liquefied gasstream is expanded to a pressure suitable for transportation of theliquefied gas. Vapors produced by expanding the liquefied gas to thetransporting pressure, along with vapor boiling-oil of the receivingvessel, are added to a methane refrigeration cycle. In the methanerefrigeration cycle, methane vapors are compressed and cooled, and atleast a portion of the compressed vapors are expanded through awork-producing zone to produce a low-temperature refrigerant which inturn is used for liquefying the natural gas stream. The presentinvention also contemplates the use of a nitrogen stripping tower forremoving nitrogen from the natural gas feed stream, wherein the removednitrogen is also utilized in the liquefaction of the natural gas feedstream. Therefore, the refrigerants for cooling the natural gas streamto a liquefied state are obtained from the original natural gas feedstream, to greatly simplify the supply prob lems involved in operationof a commercial liquefaction system. All of the refrigerants used in asystem practicing this invention may be obtained from the natural gasfeed stream. This invention further contemplates the use of a portion ofthe methane refrigerant for maintaining the desired temperature in thenitrogen stripper, to utilize the maximum amount of refrigerationdeveloped by the method, and to reduce energy requirements.

An important object of this invention is to provide a method ofliquetying gas, wherein the refrigerants used in liquefying the gas areobtained from the gas feed stream.

Another object of this invention is to remove nitrogen from a naturalgas feed stream during the liquefaction of the feed stream, and utilizethe refrigeration of the removed nitrogen in the liquefaction of thefeed stream.

Another object of this invention is to provide a novel expander cyclefor methane vapors in a natural gas liquefaction system to providerefrigerants which are in turn used in the liquefaction of the gas.

A further object of this invention is to minimize the energy required inthe liquefaction of a natural gas feed stream.

A still further object of this invention is to provide a method ofliquefying gas which requires the use of a minimum amount of equipment,and wherein a system practicing the invention will have a long servicelife and will require a minimum of maintenance.

Other objects and advantages of the invention will be evident from thefollowing detailed description, when read in conjunction with theaccompanying drawing which illustrates this invention.

The single figure of the drawing is a flow diagram illustrating apreferred practice of the present invention.

The process will hereinafter be described in detail with reference tothe liquefaction of natural gas at a source of supply, using anoperative set of temperature and pressure conditions. We want itunderstood, however, that the conditions set forth are merelyillustrative and may easily and properly be varied in consonance withthe design and capacity of the apparatus, the character of the gas fromthe standpoint of composition, temperature, and pressure, and theconditions under which the liquefaction is carried out as influenced bythe volume of material, types of refrigerants and the like, all withinthe scope of the invention. In the example, the gas to be liquefied willbe a natural gas from which the moisture, acid gases such as carbondioxide, hydrogen sulfide,

a and the like, will previously have been removed by pretreatment in theform of desiccators, amine extractors, and the like. In this typicalexample, a cleaned natural gas is used having about 73 mol. percentmethane, about 12 mol. percent ethane, about 8 mol. percent propane, andabout 2 mol. percent nitrogen, with the remainder being minorpercentages of heavier hydrocarbons. It

will be understood that natural gas capable of being processed inaccordance with the teachings of this invention may have up to 2\ 25mol. percent heavier hydrocarbons, up to 20 mol. percent nitrogen, andup to 5 mol. percent carbon. dioxide or hydrogen sulfide, but usuallythe amount of methane will be from about 70 "to about 90 mol. percentof-the natural gas feed stream.

Referring to the drawing in detail, reference character 2 designates aline/leading from the source of supply (not shown) of the natural gas tobe liquefied for conveying the natural gas in a feed or process streamto a heat exchanger 4. The natural gas will normally be available at asubstantial pressure, such as 700 p.s.i.a, and the gas may be liquefiedat whatever pressure it is available. However, it is preferred that asuitable compressor 6 be interposed in the gas feed line 2 upstream ofas the temperature of the gas is reduced, the relative humidity isincreased, raising the possibility of condensate plating out ontothe'surfaces of the heat exchanger 4.

Therefore, it is preferred that the'natural gas feed stream be passedthrough the drier after the temperature has been reduced about 105 F.for a further removal of moisture from the gas. The drier may be of anydesired construction and utilize any desired drying agent, such as,alumina or silica. The dried feed stream is fed back intothe heatexchanger through a line 34, for subsequent passage through theremainder of the heat exchangerto the outlet line .16.

The liquefied natural gas discharging from the heat exchanger 4 throughthe line 16 will be at about l65 F. and at a pressure of about 1490p.s.i.a. 'It will thus be noted that the natural gas is only slightlyreduced in pressure by passage through the heat exchanger 4 and V inbeing cooled to a liquefied state.

the heat exchanger 4 to compress the gas to a substantial 1 pressure,such as 1500 p.s.i.a. Also, it is preferred that a suitable heatexchanger 3 be interposed in the line 2 between the compressor 6 and theheat exchanger 4- for removingthe heat of compression from the naturalgas feed stream by use of a readily available high temperature levelrefrigerant, such as water.

fed through the line 12 is at about 222 F., and the re-,

frigerant fed through the line leis at about -253 F. The threerefrigerants are directed in parallel with the natural gas, such thatthe natural gas will be cooled upon passage through the heat exchanger4, and the natural gas will be in a liquefied state upon dischargingthrough the line 16 from the end of the heat exchanger 4 opposite feedline 2. It may also be noted that the heat exchanger 4 may beconstructedin two or more sections, although it is preferred toprovide one heatexchanger with three separate coils extending therethrough for passageof the three separate refrigerants in parallel with the natural gasbeing cooled. The refrigerants fed through the lineslll, 12, and 14 willbe described in detail below.

In a preferred embodiment of this invention, the natural gas iswithdrawn from the heat exchanger 4 through a line 18 after the gas hasbeen cooled about 100 F. The cooled gas is fed through the line 18 intoa knock-out drum 2t),wherein any condensates which may be present in thegas may be separated. The vapor collecting in the upper end of theknock-out drum 24) is withdrawn through aline 22 and then divided intoseparate streams through lines 24 and 26. Only a minor portion of thegas or vapor. is fed through the line 24, and this gas is used toreplenish the fuel gas supply in a line 28 which furnishes the fuel forvarious units of equipment used in a system practicing this invention.The major portion of the gas in the line 22 is directed through the line26 into a suitable drier 30. It may also be noted that the condensatescollecting in the lower end of the knockout drum 20 are dischargedthrough a line 32 to combine with the gas in the line 26 being fed tothe drier 3t).

As previously indicated, the original natural gas feed stream being fedto the heat exchanger 4 will ordinarily have been dried to a lowrelative humidity. However,

. Thus, the natural gas being fed to the heat exchanger 4 is preferablyat about The liquefied natural gas is passed from the line 16 throughthe reboiler section 36 of a nitrogen stripper tower 38. I

The tower 33 may be of any suitable construction which has the reboiler36 in the lower section thereof and a reflux condense-r 4% in the uppersection thereof for maintaining the desired temperatures in the .towerand providing a removal of nitrogen from the feed stream, as will bedescribed. The liquefied natural gas circulating through the reboiler 36maintains the contents in the lower end portion of the tower 33 at atemperature of about 199 F., with a simultaneous cooling of theliquefied natural gas to a temperature of about 195 F. The cooledliquefied natural gas is discharged from the reboiler 36 through a line42 back into the medial portionofthetower 38. However, an expansionvalve 44 is-interposed in the line 42 to reduce the pressure of theliquefied natural gas entering the tower 38 to about.

80 to 85: p.s.i.a., with a resulting reduction in temperature v of theliquefied natural gas to about -203 F.

The expansion of the liquefied natural gas into the tower 38 provides arevapor-ization of a portion of the feed stream, with all of thenitrogen in the feed stream being included'in the vapor. The refluxcondenser 40 1s mantained at a temperature below the temperature of theexpanded feed stream entering the 'to'wer 38, and preferably at atemperatureof about 253 F., to facilitate the condensation of naturalgas vapors flowing up- As previously indicated, the reboiler 36 ismaintained at a temperature above the temperature of the expandednatural gas fed to the tower, such that the reboiler 36 will facilitatethe vaporization of the nitrogen component'in the feed stream. Thecombination of the reboiler as and reflux condenser 40 providesnitrogen-enriched vapors in the upper end of the tower and substantiallynitrogen-free liquid in the lower end of the tower. In the example usedfor illustration, the vapor in the upperend of the tower 38 willconsistsof about mol. percent methane and about 25 mol. percentnitrogen; whereas, the liquid in the lower end of the tower 38 willconsist only of methane and heavier hydrocarbons, with no nitrogen 1being present.

The nitrogen-enriched vapors in the upper end of the tower 38 arewithdrawn. through the line 12 and fed to one of the sets of coils ofthe heat exchanger 4 to provide a portion of the cooling of the naturalgas feed strearn, as previously described. The temperature of these nitrogen-enriched vapors will be about 222 F. upon entering the heatexchanger 4 and the vapors will be at a pressure of about 85 p.s.i.a.After passage through the heat exchanger 4, the nitrogen-enriched vaporswill be at a temperature of about F. and are fed through the cooler Si)is interposed in the line 46 and is maintained at a temperature of about-253 F., as will be described, to subcool the liquefied natural gas to atemperature of about 246 F The pressure of the subcooled liquefiednatural gas will be about 80 p.s.i.a. The liquefied natural gas flowingthrough the line 46 is then expanded through a suitable expansion valve52 down to about atmospheric pressure, or slightly above, such as 17.7p.s.i.a. The expansion valve 52 is operated by a suitable liquid levelcontroller 54 mounted on the side of the. stripper 38 to control theliquid level in the stripper. By expanding the subcooled liquefiednatural gas from about 80 p.s.i.a.

down to about 17.7 p.s.i.a., the liquefied natural gas is not cooled toany appreciable extent, with the temperature of the liquefied naturalgas in the storage vessel 48 being about the same as the temperature ofthe liquefied natural gas discharging from the subcooler 50. It willalso be noted that since the liquefied gas is subcooled, a minor amountof vapor will be flashed off by reducing the pressure to about 17.7p.s.i.a.

As will be apparent to those skilled in the art, the storage vessel 43cannot be perfectly insulated; there fore, at least a minor portion ofthe liquefied natural gas in the storage vessel will boil oif as a vaporwith a temperature slightly higher than the temperature of the liquid inthe storage vessel. In accordance with the present invention, thisboil-oil vapor is withdrawn through a line 56 and added to methanerefrigerant vapors being passed through a series of compressors 58through 62. These methane refrigeration vapors may be obtained from thenatural gas feed stream when the system is first started up, and theboil-01f vapors are used as make-up in the refrigeration cycle.

The compressors 58 through 62 progressively compress the methanerefrigerant vapor until the pressure of the vapor is about 1500 p.s.i.a.Also, suitable watercooled heat exchangers or intercoolers 63 through 67are interposed between the various compressors and at the discharge endof the compressor 62 to remove all, or substantialy all, of the heat ofcompression from the methane vapor.

In a typical installation, the compressor 53 Will increase the pressureof the methane vapor from about 16 p.si.a. to about 53 p.s.i.a., with anincrease in temperature of the vapor from about 23 F. to about 223 F.The intercooler 63 reduces the temperature of this vapor to about 1005., with a resulting pressure drop to about 48 p.s.i.a. The compressor59 increases the pressure of the methane vapor to about 154 p.s.iia.,with a resulting temperature rise to about 324 F. It may also be notedthat the compressors 58 through 62 are preferably multi-stagecompressors to facilitate the Withdrawal of vapor from the compressionsystem at substantially any desired pressure. For example, a portion ofthe methane vapor is withdrawn from the compressor 59 at a pressure ofabout '70 p.s.i.a. and fed through a line 68 for use as make-up fuel inthe line 28. The intercooler 64 cools the remaining compressed methanevapors to about 100 F, with the pressure of the vapors being reduced to149 p.s.i.a. by passage through the cooler 64. The compressor 60increases the pressure of the vapor to about 294 p.s.i.a., with aresulting temperature rise to about 165 F. The remaining intercoolers65, 66, and 67, and the remaining compressors 61 and 62 operate insubstantially the same manner to provide the methane refrigerant vapordischarging from the last cooler 67 with a pressure of about 1500p.s.i.a. and a temperature of about 100 F.

The methane vapor is then divided in two separate streams by lines 70and 72. In the example being taken for illustration, approximately twiceas much of the methane vapor is directed through the line 70 as throughthe line 72. The methane vapor passing through the line 70 is chilleddown to a temperature of about -l F. by passage through a propanechiller 74. Simultaneously,

the methane vapor passing through the line 72. is chilled down to about30 F. by a propane chiller '76. As indicated, the refrigerant used inthe chiller 74 and 76 is preferably propane which may be easilycirculated through a closed propane refrigeration cycle (not shown) tomaintain the temperature of the propane fed to the chiller 74 at atemperature of about 20 F. and the propane fed to the chiller 76 at atemperature of about 20 F. Al though the details of the propanerefrigeration cycle form no part of the present invention, it should benoted that the propane refrigerant may be easily obtained from thenatural gas feed stream, such that the refrigerant supply in acommercial installation will be substantially unlimited, and the supplyproblem for the installation will be minimized. However, any commercialrefrigerant, such as Freon, may be used in the chillers 74 and 76 ifdesired.

The methane vapor passing through the line 70, after being chilled to l0F., is expanded through any suitable device 73 which will form aWork-producing zone and derive work or energy from expansion of thevapor. For example, the device 78 may be a turbine, such that the energytaken from the shaft of the turbine will be derived from the expansionof the vapor through the turbine. It is preferred that the dischargepressure of the expander 73 be set at about 214.9 p.s.i.a., such thatthe temperature of the vapor discharging from the expander 73 will be atabout 175 F. This expanded methane vapor is discharged from the expander78 through the line lit to serve as the lower temperature refrigerant inthe cooling of the natural gas feed stream passed through the heatexchanger 4, as previously described.

The refrigerant passed through the exchanger 4 from the line 10 will beheated to about 80 F. by heat exchange with the natural gas feed stream.This heated refrigerant is withdrawn from the heat exchanger 4 through aline 80 and directed into the compressor portion of the cycle at thecompressor 60. It may be noted that this refrigerant will have beenreduced slightly in pressure from 214.9 p.s.i.a. to about 212 p.s.i.a.by passage through the respective coils of the heat exchanger 4, suchthat the pressure is compatible with the pressure of the vapor at anintermediate stage of the compressor 60. It should also be noted that byexpanding this portion of the vapor down to an intermediate pressurebetween the pressure of the methane vapor before and after the completecompression cycle, this portion of the vapor only needs to berte-compressed partially through the compressor 60 and through thecompressors 61 and 62 before being re-expanded and re-used.

The methane vapor chilled by the chiller 76 is in turn subdivided intotwo separate streams by a pair of lines 82 and 34. In the example takenfor illustration, about twice as much vapor is directed through the line84 as is directed through the line 82. As previously noted, the methanevapor downstream of the chiller 76 will be at about 30 F. for passagethrough the lines 82 and 84. That portion of the vapor passing throughthe line 32 is condensed by passage through a heat exchanger 86. Therefrigerant for the heat exchanger or condenser 86 is obtained byexpanding the portion of the methane vapor in the line 84 through asuitable expander 88. The expander 83 may take any desired form whichwill provide a work-producing zone for the expansion of the vaportherethrough. The discharge of the expander 88 is set at about 19p.s.i.a., such that the expanding vapors will be decreased intemperature to about 253 F. The expanded and cooled methane vaporsdischarging from the expander 38 are directed through a line 90 to thecon denser 86 for removing heat from the portion of the methane vaporsdirected through the line 82, such that the temperature of the expandedvapors downstream from the condenser 86 will be at about 15 F. and theternperature of the condensed methane refrigerant discharging from thecondenser 86Wi1l be at about 243 F.

The heated methane vapors discharging from the condenser 85 on throughthe line 96 are directed back to the line 56 for're cycling through thecompressors 58 through The condensed methane refrigerant flowing throughthe line 82 from the condenser 86 is subdivided into two streams anddirected through lines 92 and $4. In the example taken for illustration,approximately twice as much of the methane is directed through the line94 as through the line 92. Each of the'lines 92 and 94 contains asuitable expansion valve 96 to reduce the pressure of the methane,refrigerant flowing through both of the-lines 92 and 94 to about 19p.s.i.a., with a partial revaporization ofthe methane and a resultingtemperature drop to about The methane refrigerant flowing through theline 92 is directed through the reflux condenser 40 in the uppersectionof'the tower 38 to maintain the. temperature of the condenserbelow the temperature of the expanded liquefied natural gas fed to thetower, as previously described. The methane refrigerant passing throughthe reflux condenser 46 Will pick up sufiicient heat to complete therevaporization thereof and is withdrawn through the line '14 to form thelowest temperature refrigerant directed through the heat exchanger 4.The methane refri erant directed through the line 94 is passed throughthe subcooler t? to subcool the liquefied natural gas dischargingfrornthe bottom of the tower 58, as previously described. This portionof the methane refrigerant willalso pick up sufiicient heat, by passagethrough the subcooler 56 to complete the revaporization thereof, and thevapor is then directed back to the line .114 to combine with thevapordischarging from the reflux condenser 49 and form the final refrigerantin the line 14. e 1

" "The refrigerant fed to the heat exchanger 4- through the line 14 isincreased in temperature to about 80 F. by

heat exchange with the natural gas feed stream, with a a resultingdecrease in temperature of the natural gas feed stream until the naturalgas is liquefied, as previously described. This last-mentionedrefrigerant is withdrawn from the respective coils of the heat exchanger4, fed through a line 98 and then line 56 for re-cycle through thecompressors 53 through 62. Thus, all of the methane refrigerant isre-cycled through the compressors to form a closed methane refrigerationcycle. 7

From the foregoing it will be apparent that the present inventionprovides a novel method of liquefying natural gas, wherein the necessaryrefrigerants are obtained from the natural gas feed stream and need notbe acquired from an outside source. Any nitrogen which maybe present inthe natural gas feed stream is efficiently removed and [then usedasarrefrigerant in the liquefaction of the feed stream; whereupon theremoved nitrogen may be used in a fuel for various units of equipmentrequired in a system practicing the invention. It will be furtherapparent that the natural gas feed stream is first liquefied at anelevated pressure to take advantage of using refrigerants at relativelyhigh temperature levels, with the liquefied natural gas beingsubsequently expanded into a suitable storage vessel. 7 The boil-01ffrom the storage vessel is added to a methane refrigeration cyclewherein the methane is compressed and cooled, and at least a portion ofthe compressed methane is expanded through a workrangement of steps andprocedures as, heretofore set forth a in the specification and shown inthe drawing, it being understood that changes may be made in the preciseem,- bodiment disclosed without departing from thespirit and scope ofthe invention, as defined in thefollowing claims. For example, thepreferred method involves the withdrawal of the natural gas feed streamfrom an intermediate portion of the heat exchanger 4 for make-up fueland for passage'o'f the feed stream through the drier 30 to lower therelative humidity of the stream and prevent plat ing of condensates onany of the coils orexposed surfaces of the heat exchanger 4. However, ifsuflicient fuel is available from the nitrogen stripper 38 and theboil-off vapor,the knock-out drum Zilmay be eliminated and the naturalgas feed stream passed directly from the line 18 through the drier 30..Also, that portion of the methane refrigerant vapor passed through theline 82 andcondensed by the condenser -may be taken from the line 70rather than the line "72 if desired. In fact, this portion of themethane vapor will then be at lower temperature to facilitatecondensation thereof by passage through the condenser 85, although therefrigerationiload of the sepa-' rate propane refrigeration cycle willbe increased by the necessity of'increasing the amount of methane vapordirected through the lower temperature chiller 74. We claim: a 1. In themethod of liquefying natural gas available in a stream at a pressureabove atmospheric, the steps of: V (a) cooling the natural gas stream toa liquid without a substantial reduction in the pressure of the stream,-.(b) expanding the liquefied gas into a storage vessel,

(0) compressing and cooling methane vapor, a portion of which isrecovered from the vapor zation of liquid gas contained in the storagevessel, 7 (d) expanding at least a portion of the cornpressed methanevapor through a work producing zone to produce refrigerant having atemperature below the initial temperature of the natural gas stream, and

(e) passing said refrigerantin heat exchange relation with the naturalgas stream to obtain at least a portion of the cooling called for instep (A); characterized further in dividing the compressed andcooled'methane vapor into first, second, and third separate streams;expanding the first separate stream through relation with the naturalgasstream to obtain a portion producing zone to form a refrigerant for heatexchange With the natural gas feed stream. Also, in the preferredembodiment, the compressed methane vapor is initially subdivided intotwo separate streams, with one of the streams being passed through aWork-producingzone for the formation of a relatively high temperaturelevel refrigeraut used-in cooling the natural gas feed stream, and theother methanervapor stream being in turn subdivided for a partialpassage through a work-producing zone in the formation of alowtemperature level refrigerant for cooling and liquefaction of thenatural gas feed stream,

Changes may be made in the combination and arof the cooling :called forin step (a); expanding the second separate stream through awork-producing zone to a temperature below the condensation temperatureof the third separate stream; passing the expanded second separatestream in heat-exchange relation with the third separate streamtocondense the third'separate stream; expanding the condensed thirdseparate stream to form a second refrigerant having a temperature belowthe condensation temperature of the natural gas stream, then passing thesecond refrigerant in heat-exchange relation with the natural gas streamto complete the cooling calledfor in step (a).

2. The method defined in claim 1 characterizedfurther in removingmoisture from the natural gas stream at an intermediate stage of thecooling thereof by said firs and second refrigerants. I

3." The method defined in claim 1 characterized further in" that saidfinst and second refrigerants, after passage in heat-exchange relationwith the natural gas, and said second separate stream, after passagethereof in heatexchange relation with the third separate stream, arerecompresse'dand re-cycled throughthe steps set forth in claim 4. 1

4. The method defined in claim 1 characterized fur-' ther in that themethane vapor is compressed and cooled in stages before division intosaid separatestreams, and

said first refrigerant is re-cycled to an intermediate stage ofcompression after passage in heat-exchange relation with the natural gasstream.

5. In a method of liquefying natural gas containing nitrogen andavailable in a stream at a temperature above atmospheric, the steps of:

(a) cooling the natural gas stream to a liquid without substantiallyreducing the pressure of the stream,

(1)) expanding the liquefied gas stream into a nitrogen stripping towermaintained at such temperatures as to provide nitrogen-enriched vaporsin the upper end of the tower and substantially nitrogen-free liquid inthe lower end of the tower,

(c) passing the nitrogen-enriched vapors in heat-exchange relation withthe natural gas stream to obtain a portion of the cooling called for instep (a),

(at) expanding the nitrogen-free liquefied gas into a storage vessel,

(e) compressing and cooling methane vapor, at portion of which isrecovered from the vaporization of liquid gas contained in the storagevessel,

(f) expanding at least a portion of the compressed methane vapor througha work-producing zone to produce a refrigerant, and

(g) passing said refrigerant in heat-exchange relation with the naturalgas stream to obtain an additional portion of the cooling called for instep (a);

characterized further in removing moisture from the natural gas streamafter the stream has been cooled about 100 F.

6. In a method of liquefying natural gas containing nitrogen andavailable in a stream at a temperature above atmospheric, the steps of:

(a) cooling the natural gas stream to a liquid without substantiallyreducing the pressure of the stream,

(b) expanding the liquefied gas stream into a nitrogen stripping towermaintained at such temperatures as to provide nitrogen-enriched vaporsin the upper end of the tower and substantially nitrogen-f ee liquid inthe lower end of the tower,

(c) passing the nitrogen-enriched vapors in heat-exchange relation withthe natural gas stream to obtain a portion of the cooling called for instep (a),

(d) expanding the nitrogen-free liquefied gas into a storage vessel,

(e) compressing and cooling methane vapor, a portion of which isrecovered from the vaporization of liquid gas contained in the storagevessel,

(7) expanding at least a portion of the compressed methane vapor througha work-producing zone to produce a refrigerant, and

(g) passing said refrigerant in heat-exchange relation with the naturalgas stream to obtain an additional portion of the cooling called for instep (a);

characterized further in that the methane vapor is compressed and cooledin stages; the compressed methane vapor is subdivided into a first, asecond, and a third separate stream; expanding the first separate streamthrough a work-producing zone to an intermediate pressure between thepressure of the methane vapor before and after compression to form afirst refrigerant; passing the first refrigerant in heat-exchangerelation with the natural gas stream to obtain a portion of the coolingcalled for in step (a); expanding the second separate stream through awork-producing zone to a temperature level below the condensationtemperature of the third separate stream; passing the expanded secondseparate stream in heat-exchange relation with the third separate streamto condense the third separate stream; expanding the condensed thirdseparate stream to produce a second refrigerant having a temperaturelevel below the condensation temperature of the natural gas stream; thenpassing the second refrigerant in heat-exchange relation with thenatural gas stream to complete the cooling called for in step (a).

7. The method defined in claim 6 characterized further in re-cycling thefirst refrigerant to an intermediate state of compression of the methanevapor.

8. The method defined in claim 6 characterized further in that thenitrogen stripping tower has a reflux condenser in the upper sectionthereof maintained at a temperature below the temperature of theexpanded liquefied 1 natural gas and a reboiler in the lower sectionthereof maintained at a temperature slightly above the temperature ofthe expanded liquefied natural gas, and said reboiler is maintained atsaid temperature by passing the liquefied natural gas streamtherethrough prior to expension of the stream into the tower.

9. The method defined in claim 8 characterized further in that thereflux condenser is maintained at the same temperature by passing aportion of the second refrigerant therethrough prior to passage of thesecond refrigerant in heat-exchange relation with the natural gasstream.

10. The method defined in claim 9 characterized further in passing theremainder of the second refrigerant in heat-exchange relation with theliquefied natural gas Withdrawn from the lower end of the tower beforepassage of the second refrigerant in heat-exchange relation with thenatural gas stream to subcool the liquefied natural gas prior toexpansion thereof into the storagevessel.

11. In the method of liquefying natural gas available in a stream at apressure above atmospheric, the steps of:

(a) cooling the natural gas stream to a liquid without a substantialreduction in the pressure of the stream,

(12) expanding the liquefied gas into a storage vessel,

(c) compressing and cooling methane vapor, a portion of which isrecovered from the vaporization of liquid gas contained in the storagevessel,

(d) expanding at least a portion of the compressed methane vapor througha work-producing zone to produce refrigerant having a temperature belowthe initial temperature of the natural gas stream, and

(e) passing said refrigerant in heat exchange relation with the naturalgas stream to obtain at least a portion of the cooling called for instep (:1);

characterized further in condensing another portion of the compressedmethane vapor, expanding said condensed methane vapor to form a secondrefrigerant having a temperature below the condensation temperature ofthe natural gas stream at the initial pressure of the stream, andpassing said second refrigerant in heat-exchange relation with thenatural gas stream to obtain an additional portion of the cooling calledfor in step (a).

References Cited in the file of this patent UNITED STATES PATENTS KnappNov. 8,

1. IN THE METHOD OF LIQUEFYING NATURAL GAS AVAILABLE IN A STREAM AT APRESSURE ABOVE ATMOSPHERIC, THE STEPS OF; (A) COOLING THE NATURAL GASSTREAM TO A LIQUID WITHOUT A SUBSTANTIAL REDUCTION IN THE PRESSURE OFTHE STREAM, (B) EXPANDING THE LIQUEFIED GAS INTO A STORAGE VESSEL, (C)COMPRESSING AND COOLING METHANE VAPOR, A PORTION OF WHICH IS RECOVEREDFROM THE VAPORIZATION OF LIQUID GAS CONTAINED IN THE STORAGE VESSEL, (D)EXPANDING AT LEAST A PORTION OF THE COMPRESSED METHANE VAPOR THROUGH AWORK PRODUCING ZONE TO PRODUCE REFRIGERANT HAVING A TEMPERATURE BELOWTHE INITIAL TEMPERATURE OF THE NATURAL GAS STREAM, AND (E) PASSING SAIDREFRIGERANT IN HEAT EXCHANGE RELATION WITH THE NATURAL GAS STREAM TOOBTAIN AT LEAST A PORTION OF THE COOLING CALLED FOR IN STEP (A);CHARACTERIZED FURTHER IN DIVIDING THE COMPRESSED AND COOLED METHANEVAPOR INTO FIRST, SECOND, AND THIRD SEPARATE STREAMS; EXPANDING THEFIRST SEPARATE STREAM THROUGH A WORK-PRODUCING ZONE TO FORM A FIRSTREFRIGERANT HAVING A TEMPERATURE BELOW THE INITIAL TEMPERAURE OF THENATURAL GAS STREAM; PASSING SAID FIRST REFRIGERANT IN HEAT-EXCHANGERELATION WITH THE NATURAL GAS STREAM TO OBTAIN A PORTION OF THE COOLINGCALLED FOR IN STEP (A); EXPANDING THE SECOND SEPARATE STREAM THROUGH AWORK-PRODUCING ZONE TO A TEMPERATURE BELOW THE CONDENSATION TEMPERATUREOF THE THIRD SEPARATE STREAM; PASSING THE EXPANDED SECOND SEPA-